With the advent of increasingly stringent environmental regulations, the amount of volatile organic compounds (VOC's) present in liquid paint materials has come under increasingly stringent controls regarding both the nature and the quantity of such VOC emissions. Aromatic solvents such as benzene, toluene and xylene, some of the more powerful solvents for liquid coatings, have been highly restricted, particularly in industrial environments, because of their suspected carcinogenic nature and the contribution such solvents make to photochemical smog and other air pollution problems in an urban environment. As a consequence of such restrictions, the coating industry has increased its use of high solids coatings in order to lower VOC emissions. In general, the coatings industry regards a low VOC air dry coating as one which contains less than about 3.5 lbs. of solvent per gallon of coating. For some applications in some areas of the country, the figure may be as low as 2.8 lbs. of solvent per gallon.
There have been numerous proposals for the manufacture of high solids coating compositions. For example, U.S. Pat. No. 4,397,989 to Adesko is directed to a high solids coating composition which contains at least 40% by weight of a binder of film-forming constituents in which the constituents are: (1) an acrylic polymer of styrene or methylmethacrylate or mixtures thereof, an alkyl acrylate or an alkyl methacrylate and a hydroxyl alkyl acrylate or methacrylate (2) a polyester polyol and (3) an alkylated melamine formaldehyde cross-linking agent and optionally, an acid catalyst. The composition is particularly useful, as a primer, or an exterior finish for automobiles, trucks and airplanes, and can be used as an appliance finish and for coil coatings.
U.S. Pat. No. 4,477,534 to Antonelli et al. discloses air-drying resins such as acrylate- or methacrylate-acid drying-oil resins and alkyds which can be blended with up to 70% by weight of the combined weights of a vinyl oxazoline ester as a reactive diluent. In an organic solvent, the blend produces a suspension or solution which has a lower viscosity than a suspension or solution of resin alone but with the same percentage of solids. When applied to a substrate, the blend dries in air to a good protective and ornamental coating. The oxazoline ester can be combined with up to about twice its weight of certain melamine condensates.
U.S. Pat. No. 4,166,054 to Meeske et al. discloses an aqueous system which includes an epoxy resin copolymer which is an air curable resin solution useful for surface coating and impregnation comprising (I) the reaction product of (A) from about 50% to about 65% by weight based upon the total weight of (A) and (B) of an epoxy resin ester of a partially conjugated unsaturated fatty acid and (B) from about 50% to about 35% by weight based upon the total weight of (A) and (B) of a blend of reactive monomer possessing reactive double bonds, at least one of which must be an unsaturated mono-basic acid in the presence of (II) an alcohol ether of a glycol and subsequently reacted with (III) an amine and then (IV) dispersed in water, wherein the epoxy resin of component (A) is the reaction product of 2,2,bis(4 hydroxyphenyl) dimethylmethane and epichlorohydrin, and has a melting point from about 130.F. to about 230.F. and an epoxide equivalent weight within the range from about 400 to about 1100, and the fatty acids used are straight chain monobasic acids of 18 carbon length having double bonds arranged in the chain in an amount and position to give an iodine number of 125 to 185, an acid number of from about 180 to 210 and a percentage of conjugation of the double bonds between 20% and 25% wherein the reaction is carried out to an acid number below 10 and the monomer (B) portion consists of a mixture of 20-28% of unsaturated monobasic acids having a polymerizable double bond and 80% to 72% reactive monomers having a polymerizable double bond.
U.S. Pat. No. 4,224,202 to Heiberger discloses a high-solids coating composition containing an unsaturated fatty acid alkyd resin, an alkyl dimethacrylate or trimethacrylate monomer, a protected cobalt oxime catalyst, and a polar solvent. The di- or trimethacrylate monomer cross-links with the alkyd and becomes part of the solid paint film.
High solids resins which have the desired low VOC characteristics have sometimes been obtained by lowering the copolymer molecular weight and by using stronger solvents to form a more solubilized coating system. In order for such a system to be commercially successful, it is necessary to maintain the drying rate and film properties associated with higher molecular weight copolymers used in solvent-resin coatings. This invention relates to the preparation of epoxy ester copolymer resins which retain the drying rate and film properties associated with the higher molecular weight copolymers and which are suitable for use in the formulation of low VOC coatings.